Reinforcing of tower base in existing guyed Towers

ABSTRACT

A method of reinforcing the tower base in existing guyed towers. A guyed tower is a tower supported by a base and guy cables. The tower&#39;s foundations are Base and Deadman anchors. The base is an isolated spread footing which is pier and pad or pile foundation. This method of reinforcing is a new mechanism to transfer part of the download on base to an alternative mat foundation. 
     The reason of using this mechanism is that the soil underneath the tower base is getting high stresses above its limit. The soil is getting overstressed due to some reasons that it happens frequently. 
     The standard procedure is before building any tower, the Geotechnical engineer does test boreholes at the proposed location of the tower foundations (Base Deadman Anchors). The geotechnical engineer provides the soil report with soil parameters at each borehole. The structural engineer designs the guyed tower and get the download on the tower base to design the spread footing (or pile). The download is the result of the dead load (tower own weight, mounts, antennas, microwave dishes, cables &amp; equipment) and live load (wind &amp; ice). The structural engineer designs the base to transfer the download from the tower to the soil safely based on the information that was given by the geotechnical engineer. In the future, if no change in loading, then the base will function well as designed, however the client always changes the dead load (antennas, microwave dishes, cables &amp; equipment) and this is due to the fast-growing telecom technologies and the country needs. So, in most cases the base requires reinforcing. 
     The nonstandard procedure is to design the tower base, based on normal dry soil. Normal dry soil is a soil with average soil parameters values, this could happen due to short notice given by the client to the geotechnical engineer and the geo couldn&#39;t make it before designing the base. In this case the structural engineer designs the base with some assumed soil parameters and increasing the download to cover the absence of soil report (multiply download by factor more than 1.0, usually 1.20). Later after receiving the soil report, sometimes the soil parameters are weaker than the parameters that was used by the structural engineer. Here, the engineer must find the stresses on soil underneath the tower base and compare it with the soil report, if it was above the soil limit then reinforcing the base is required. 
     Something else which is the ice thickness, and this is defined by the ANSI-TIA-222. Telecommunications Industry Associations reviews the ice records and publish new list of the past 50 years of ice thickness. Also, they provide some changes on ice calculation formula vs height, this all increases the download, then increases the stresses on soil. If the stresses on soil are less than the capacity of soil, then it is safe but if not then reinforcing is required. 
     By-law, clients (tower owners) are required to provide structural analysis for the tower and foundations for any loading change. Most of cases the foundation is getting overstressed, and it requires reinforcing. The new mechanism is taking part of the load that is overstressing the soil underneath the base and transfer it to a new mat foundation.

TECHNICAL FIELD

Structural Analysis and Reinforcing of tower base in existing guyed Towers.

BACKGROUND OF THE INVENTION

Reinforcing of guyed tower base have long been known and widely used when the loads increase above the soil, and stresses becoming higher than the capacity of soil. Engineers used to propose new concrete pad, connected to the existing pier (or pile) through inserting dowels into the exiting pier (or pile). The procedure of this reinforcing is to remove the soil up to the founding depth with the proposed footprint, drilling holes into the pier (or pile) all around, inserting dowels into these holes and adding adhesive material to make the dowels as part of the pier (or pile) structure. Then laying rebar mesh top and bottom around the pier (or pile), placing concrete, after concrete dry, backfill to the ground surface. This way of reinforcing is assuming that the concrete of existing pier (or pile) has not changed from the “as new” condition, and the pier (or pile) is good after drilling to insert the new dowels.

SUMMARY OF THE INVENTION

The guyed tower is an assembly of sections; these sections are connected to each other through splice connections. In this invention, the new reinforcing mechanism is carrying the tower from underneath these splice connections by using rigid frame. The rigid frame is supported on lattice legs, these legs are connected at the bottom with movable plates, the movable plates are resting on mat foundation. A steel wire cable is going around all movable plates. A turnbuckle is placed on the steel wire cable. By torquing the turnbuckle, this will result of rising lattice legs then transfer part of the download to the new mechanism. This torque will produce forces into the legs to carry the extra load.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a guyed tower, showing Base (B), Deadman anchors (D) & section height H.

FIG. 2 is showing two types of foundations, spread footing & pile foundation.

FIG. 3 is showing two sections of the tower & part of the existing base.

FIG. 4 is showing what in FIG. 3 plus adding the assembly of new mechanism.

FIG. 5 is showing all assemblies of the new mechanism.

FIG. 6 is showing assembly I.

FIG. 7 is showing assembly II.

FIG. 8 is showing assembly III.

FIG. 9 is showing assembly IV.

FIG. 10 is showing the structural system of reinforcing mechanism.

FIG. 11 is showing how to calculate the required tension then torque.

DETAILED DESCRIPTION

The guyed tower (FIG. 1 ) is a tower supported by tower Base (No. B) and guy cables which supported by Deadman anchors (No. D). The guyed tower consists of sections as shown in (FIG. 1 ), each section with height (H). Taking the first two sections as shown in (FIG. 3 ) to illustrate the existing details of the tower sections before adding the new mechanism. The pivot (No. 1.3) is the center of the tower which allows the tower to rotate only around it but does not allow the tower to move horizontally. This pivot is welded to the tower base plate, this base plate is fixed into the concrete of the pier (or pile) structure. First section of the tower has a circular base plate (No. 1.4) at the bottom and all legs (No. 1.5) are welded to it.

The tower base will be one of two types, isolated spread footing (FIG. 2 Top), pier (No. 1.2), and pad (No. 1.1) or pile foundation (FIG. 2 Bottom) (No. 1.2).

The new mechanism is consisted of (4) assemblies.

Assembly (I) (FIG. 4, 5 & 6 ) is a rigid frame; the rigid frame consists of weldments (No. I.1) and built-up beams (No. 1.2). Each weldment is located underneath the bottom of top splice pads (No. 1.6) of first tower section. Beams are welded to the weldments and forming closed frame. The weldments are connected to the existing splice pads using longer bolts to accommodate the new thickness. The built-up beam to be welded after installing all weldments on existing splice connections.

Assembly (II) (FIG. 4, 5 & 7 ) consists of lattice legs. Each lattice leg (No. II) is connected at the top to the rigid frame (No. I) using hinges and connected at the bottom to the movable plate (No. 111.3). Before connecting the lattice leg at the bottom with the movable plate, the movable plate should be inserted first into the fixed base plate (No.III.2) after adding grease in between these two plates.

Assembly (III) (FIG. 4, 5 & 8 ) consists of mat foundation (No.III.1) and base plates (FIG. 4, 5 & 8 ). The top of mat foundation is above the ground surface by few inches to keep the water away from it. Base plates are two plates at each lattice leg, one on top of the other. The bottom plate (III.2) is fixed into the concrete using concrete anchors. The top plate is movable (III.2), sliding on bottom fixed plate using grease in between.

Assembly (IV) (FIG. 4, 5 & 9 ) is a steel wire cable (No. IV.1) with a wire rope thimble (No. IV.2) at each end, this assembly of steel wire cable is going around all movable base plates. A turnbuckle (No. IV.3) is connecting the two ends of the wire assembly. Tightening the turnbuckle to set the wire assembly around the movable plates and ready to apply the required torque.

After all set, apply the required torque to produce tension in the steel wire cable, this tension will produce forces in lattice leg to carry the extra load on the base. This mechanism is to move some of the extra loading to the new mat foundation.

The structural system that is illustrating this mechanism is shown in (FIG. 10 ). To calculate the torquing required (FIG. 11 ), start by putting a unit force at the top, then calculate T. Based on the actual force from the tower analysis and how much the existing base could carry, then the extra loading on top of the new mechanism is defined, then the tension in the steel cable is defined and the required torque as well.

DEFINITIONS

B Tower Base.

D Tower Deadman Anchors.

H Section height.

T Tension force due to unit force at the top.

1.1 Concrete pad in spread footing.

1.2 Concrete pier or pile foundation.

1.3 Pivot on top of tower base.

1.4 Bottom base plate of first tower sections.

1.5 1^(st) section in the tower.

1.6 Top splice plate of 1S^(t) tower section.

1.7 2^(nd) section in the tower.

I.1 Weldment.

1.2 Beam.

II Lattice legs.

III.1 Mat foundation.

111.2 Fixed base plate.

111.3 Movable base plate.

IV.1 Steel wire cable

IV.2 Wire rope thimble.

IV.3 Turnbuckle. 

1-11- (canceled) 12-A method of reinforcing the tower base in existing guyed towers, it could be used for any kind of tower base, being spread footing or pile foundation, it is using the existing splice pad connections between first and second section of the guyed tower to install the new reinforcing assembly. 13-This method of reinforcing is a new mechanism to transfer part of the download on existing base to an alternative mat foundation, it is for any guyed tower that it is built from sections, connected to each other through splice pads (Flanges)and bolts. 14-The new mechanism consists of (4) assemblies (I, II, III & IV). First assembly (I) is a rigid frame, this frame is built around the tower and located under the top splice pad of first section. This frame is connected to the splice pads, using the same bolts with different grip. The frame is built from built-up beams and weldments, all are welded together. Second assembly (II) are lattice legs. Third assembly (III) includes mat foundation and base plates. The new mat foundation is built around the existing base at the ground surface, reinforced with rebar top & bottom each way. The base plates are two plates, one is sliding on the other. The bottom plate is fixed into the mat foundation using concrete anchors. The top plate is movable, sliding on bottom plate using grease in between plates. Fourth assembly (IV) includes a steel wire cable, a turnbuckle, wire rope thimbles & hinges. This assembly is going around all the movable base plates in assembly (III). Torquing the turnbuckle will move the movable plate towards the tower, which is connected to the lattice leg. This will result, moving part of the download from the existing base to the new mechanism. 